U.S. EPA orders the defense contractor to pay for a system to clean up a shallow aquifer in the San Gabriel Valley.

The U.S. Environmental Protection Agency has ordered defense contractor Northrop Grumman to pay $20 million for a system to clean up contamination in shallow groundwater that originated years ago from a factory in the City of Industry.

Northrop Grumman will install wells and a treatment plant to contain the contamination, according to an EPA statement released Wednesday. The treated water will be discharged back into the underground aquifer.

The water was contaminated by a now-defunct electronics factory belonging to Benchmark Technology, a company owned for many years by TRW, an aerospace and credit firm.

TRW was ordered to clean up groundwater contaminated by several facilities it owned in the San Gabriel Valley. When TRW was bought by Northrop Grumman in 2002, the defense contractor assumed liability for the cleanup costs.

A Northrop Grumman spokeswoman did not respond Wednesday to an email seeking comment.

In 2009, Northrop agreed to pay $21 million to clean up contamination in a deeper aquifer, roughly more than 300 feet below ground, south of Puente Creek in Industry and nearby communities. The $20-million assessment is to clean up water in a separate aquifer above that.

“The shallow zone is the most contaminated,” said Ray Chavira, the EPA’s project manager for the area.

The area is one of four federal Superfund sites in the San Gabriel Valley, where more than 30 square miles of the water table are polluted with solvents and degreasing agents used for decades by business and industry.

The pollution, first detected in 1979, has affected the primary water source for more than a million residents, causing the closure of wells and spawning a long cleanup battle.

The contaminated groundwater lies under portions of 10 San Gabriel Valley cities: Alhambra, Irwindale, La Puente, Rosemead, Azusa, Baldwin Park, Industry, El Monte, South El Monte and West Covina. Some 45 water suppliers use the San Gabriel Basin groundwater aquifer to provide 90% of the drinking water for more than a million people, according to the EPA.

The Benchmark Technology site, at 200 South Turnbull Canyon Road, is one of the largest sources of volatile organic compound contamination in the Puente Valley section of the Superfund sites.

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